1. Field of the Invention
The present invention relates to variable resistors and in particular to a ceramic resistor card for use in a fuel level sensor that is lower in cost and has improved resistance to sulfur corrosion.
2. Description of the Related Art
Variable resistors are known for sensing parameters in a variety of applications. For example, the fuel level in an automobile tank is typically measured using a variable resistor having a sweep arm mechanically or electrically coupled to a float located in a fuel tank. The sweep arm position varies according to the float level. The position of the sweep arm can be detected by measuring the voltage across the variable resistor. Therefore, the voltage detected across the variable resistor is an indication of fuel level.
Examples of such fuel sensors are disclosed in the following U.S. Pat. Nos. 5,746,088, 6,021,668, 6,127,916, 6,212,950 and U.S. Patent publication number 2002/0040597.
Variable resistors used for fuel level measurement are typically a card with metalized conductor areas and thick film resistive ink. The thick film ink is deposited in precise areas to interconnect specific metallized areas. A sweep arm is pivotally mounted to the patterned card and includes a wiper blade like assembly with contact fingers. As the float raises and lowers according to the fluid level, the wiper contact fingers move along the resistor card in an arcuate path and make contact with the metalized areas. This results in a voltage change that generates a signal representative of the amount of fuel contained in the tank.
One problem with this type of system is that over the life of a vehicle the assembly must go through thousands of cycles in a harsh environment. The card is exposed to both wet and dry conditions as the fluid level is decreased. In addition, the assembly is exposed to severe vibration resulting from vehicle movement. Wear occurs as the wiper contact fingers go back and forth over the metalizations. The electrical resistance of the metalization portions may increase and cause accuracy problems with the fuel reading. Additionally, the metalized portions may wear to the point that an open circuit is created on the card.
The resistor card is manufactured by using a ceramic substrate that is screen printed with conductive and resistive inks and fired at high temperatures in a furnace. Conductive inks used in the manufacturing of the ceramic card contain both various metals and binders. Metals used in the conductive inks include silver, platinum, palladium, gold, copper, as well as others. Silver is desired for conductive properties, low cost and the ease with which electrical wires can be soldered to the substrate. Other metals such as palladium are used in the ink to provide strength against shear forces exerted by the sliding contacts and corrosion resistance. A commonly used conductor is 2.3 parts silver to 1 part palladium by weight.
Silver has many desirable properties as a conductor. Unfortunately, silver is also chemically reactive to compounds found in fuels. For example, sulfur in fuel can attack silver to form various compounds such as silver sulfide (Ag2S), which are non-conductive. These deposits of nonconductive material generate contact resistance and create shifts and/or spikes in the output signal of the fuel sensor. This increased contact resistance may appear as signal “noise” where the output “spikes” momentarily from the proper reading.
One method to improve the corrosion resistance of the resistor card is to increase the amount of gold, palladium or platinum used in the conductive metallization and decrease the amount of silver. Unfortunately, the cost of gold, palladium and platinum is about 100 times greater than the cost of silver. This results in a resistor card that is prohibitively expensive.
A current unmet need exists for a fuel tank resistor card that is both resistant to chemical attack and corrosion and that is cost effective to manufacture.